1. Field of the Invention
The present invention relates to a high frequency switch module used for switching frequency bands and switching between transmission signals and reception signals, for example, of a radio communications device such as a cellular phone, and to a multi-layer substrate used for such a high frequency switch module.
2. Description of the Related Art
Cellular phones operable in multiple frequency bands have been practically utilized. For example, cellular phones for the global system for mobile communications (GSM) started as phones operable in a single band of the extended GSM (EGSM), and has been converted to phones operable in dual bands of the EGSM and the digital cellular system (DCS), and to phones operable in triple bands of the EGSM, the DCS, and the personal communications service (PCS). The number of frequency bands in which a single cellular phone is operated is thereby increased to extend the speech channel.
Since the GSM uses a time division multiple access system, cellular phones for the GSM performs switching between transmission signals and reception signals, using high frequency switches. Many of such high frequency switches use PIN diodes as switch elements. Many of phones operable in triple bands incorporate high frequency switches with PIN diodes, too. A high frequency switch using a PIN diode is disclosed in the Published Unexamined Japanese Patent Application Heisei 11-298201 (1999).
Some cellular phones comprising high frequency switches incorporate field-effect transistors made of a GaAs compound semiconductor (hereinafter referred to as GaAs-FET) as switch elements of the high frequency switches. A high frequency switch using the GaAs-FET has advantages that the circuit is simpler, designing is easier, a reduction in size is possible, and power consumption is lower, compared to a high frequency switch using a PIN diode. The high frequency switches using the PIN diodes are used in many of cellular phones for the time division multiple access system except the GSM, such as the personal handyphone system (PHS) or the personal digital cellular (PDC) system. A high frequency switch using a GaAs-FET is disclosed in the Published Unexamined Japanese Patent Application 2002-43911.
The high frequency switches using PIN diodes have a problem that the circuit is made more complicated as the number of frequency bands to switch increases, and it takes a longer period of time to design and fabricate prototypes of high frequency switches having required characteristics. In particular, to make devices operable in the four bands of the EGSM, the American GSM (AGSM), the DCS and the PCS, or the five bands of the EGSM, the AGSM, the DCS, the PCS and the wideband code division multiple access (WCDMA), it is more difficult to design and reduce the dimensions of the high frequency switches using the PIN diodes. Moreover, if the number of frequency bands to switch increases, the high frequency switches using the PIN diodes have a problem that harmonics produced by a nonconducting PIN diode increase and a problem that a current for making the PIN diodes conducting increases, which affects the period of time for which the cellular phone is operable for speech.
On the other hand, the high frequency switch using the GaAs-FET has a problem that, when a transmission signal of large power passes through the switch, the nonlinear characteristic of the GaAs-FET causes distortion of the transmission signal which then causes harmonics of a frequency of ‘n’ times the frequency of the transmission signal, where ‘n’ is an integer equal to or greater than 2. For example, if a transmission signal of 35 dBm, the maximum value of power of a transmission signal according to the GSM standard, is supplied to the high frequency switch using the GaAs-FET, the high frequency switch produces harmonics. In some cases the magnitude of these harmonics exceed the permissible range according to the GSM standard. Cellular phones having such high frequency switches are not acceptable. Therefore, high frequency switches using GaAs-FETs are not popular among cellular phones for the GSM. As a result, a small number of high frequency switches with GaAs-FETs are used, and the yield of phones satisfying the standard is poor, which prevents a reduction in price. Consequently, the high frequency switches using GaAs-FETs have a smaller share in the market than the high frequency switches using PIN diodes.
The frequency of twice the frequency of a GSM transmission signal falls within the frequency band of DCS signals. It is therefore impossible to reject the harmonics of the frequency of twice the frequency of a GSM transmission signal by using a filter in a dual-band cellular phone operable in the GSM and DCS.
According to the GSM standard, it is required that the power of frequency components of harmonics at an antenna terminal be −32 dBm or smaller. In addition, according to the transmission standard of the GSM, it is required that the maximum power of a transmission signal at the antenna terminal be 33 to 35 dBm. Therefore, a transmission signal of about 34 dBm is typically applied to the input of a high frequency switch. Moreover, it is required that the supply voltage for operating the high frequency switch be around 2.7 volts which is the operating voltage of the cellular phone. It is desired to implement a high frequency switch of multi-branch type such as a single-pole four-throw switch, using GaAs-FETs, that satisfies the above-described requirements and is inexpensive. However, according to the high frequency switch using the GaAs-FET, harmonic components increase if the operational voltage is reduced, so that it is difficult to provide the high frequency switches using GaAs-FETs with good yields. To satisfy the above-described characteristics only by the high frequency switch using the GaAs-FETs, it is required to improve the characteristics, which needs control of the manufacturing process of the FETs, such as adjustment of pinch-off voltage of the FETs, or adjustment of the bias point of the FETs which causes distortion of waveform when a high-power input is received. It is therefore extremely difficult to satisfy the above-described characteristics only by improving the high frequency switches using the GaAs-FETs.